Implantable medical devices (IMDs) include devices designed to be implanted into a patient. Some examples of these devices include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy devices (CRTs), and devices that include a combination of such capabilities. The devices can be used to treat patients using electrical or other therapy or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include one or more electrodes in communication with one or more sense amplifiers to monitor electrical heart activity within a patient, and often include one or more sensors to monitor one or more other internal patient parameters. Other examples of implantable medical devices include implantable diagnostic devices, implantable drug delivery systems, or implantable devices with neural stimulation capability.
Additionally, some IMDs detect events by monitoring electrical heart activity signals. In CFM devices, these events can include heart chamber expansions or contractions. By monitoring cardiac signals indicative of expansions or contractions, IMDs can detect abnormally slow heart rate, or bradycardia. Some IMDs detect abnormally rapid heart rate, or tachyarrhythmia. Tachyarrhythmia includes ventricular tachycardia (VT). Tachyarrhythmia also includes rapid and irregular heart rate, or fibrillation, including ventricular fibrillation (VF).
Tachyarrhythmia also includes supraventricular tachycardia (SVT). SVT is less dangerous to the patient than VT or VF. SVT includes arrhythmias such as atrial tachycardia, atrial flutter, and atrial fibrillation. A rapid heart rate can also be due to sinus tachycardia (ST), which is a normal response to, for example, exercise or an elevated emotional state. When detected, tachyarrhythmia can be terminated with high-energy shock therapy delivered with an ICD. It is important for cardioverter defibrillators to quickly and accurately classify sensed rhythms or arrhythmias and deliver the appropriate therapy.